Air Force

F-35 ‘pushes the envelope’

The F-35 Integrated Test Force made significant progress expanding the flight envelope of the F-35A conventional takeoff and landing variant over the past few months with numerous successes in High Angle of Attack (AoA) flight test, with the completion of the first intentional departure of controlled flight Dec. 4 and now having tested the aircraft to 50 degrees AoA.

Prior to these milestones, developmental test with the F-35A was restricted to a flight envelope between -10 and 20 degrees AoA. Now, test maneuvers are being executed up to 50 AoA and intentional departures are being conducted to explore the aircraft behavior even beyond this boundary. Results will be used to clear F-35 operational aircraft to 50 AOA, directly supporting the air superiority needs of the warfighter by allowing them to aggressively maneuver the F-35A.

Throughout the High AoA testing, the F-35A’s performance has closely matched piloted simulator results and modeled predictions, giving the team the confidence in the jet to continue moving forward in the test plan.
“We are significantly matching models and it gives us good confidence in the aircraft and how to polish the flight control systems so it’s even better than what we started with. Going into this unknown area of High AoA, we really like when things match. It makes you feel very safe, although we will remain cautious all the way though,” said David Nelson, F-35 chief test pilot from Lockheed Martin.

“We don’t want a first lieutenant going through F-35 school to be the first person to see something. We, as a flight test community, feel this is a protection and a promise we must deliver to the warfighter,” he continued.

As a result of the success, the F-35 ITF has also gained momentum in delivering an envelope in 2014 to the program office to the design limit o 50 degrees AoA, along with the ability to pull 7gs throughout the envelope, and also ensuring that the jet can fly out to 700 knots and 1.6 mach.

“This is a huge milestone for the program. This is so important because in 2014, the F-35 program has made a commitment to deliver a flight envelope to the U.S. Air Force. But more than that, we are doing this so we put test pilots like “Doc” Nelson in a position where we hope no other pilot ever has to deal with. But, if they find themselves in that position, we will have seen it and have verified that they can recover the aircraft,” said Lt. Col. George Schwartz, 461st Flight Test Squadron commander.

To mitigate the inherent risk associated with the testing, the High AoA test team began taking shape more than eight years ago, made up of the best flight test professionals in their respective fields, with diverse backgrounds including the F-15 Eagle, F-16 Fighting Falcon, F-18 Hornet and F-22 Raptor.
“We have a team where every person is first-string. No one in the world today, who is testing airplanes, is more proficient at doing this than the members of the High Angle of Attack Test Team. They are the world class experts for this particular type of testing,” said Nelson.

From the team’s inception, Nelson found out he had been selected to fly the High AoA missions. For more than eight years, he has been flying the High AoA maneuvers in a simulator preparing for the opportunity to fly the missions at Edwards.

Nelson was not alone in preparing for the High AoA testing. By the time the F-35 ITF was ready to fly the first mission, the test team had already been functioning like a well-oiled machine.

“This isn’t clichÈ; the team really does function as a well-oiled machine. These people were trained very well. For many of them, training began in air start testing earlier this year. Talk about high risk, we turned our one engine off and were able to get it started again,” said Nelson.

“That was a build-up to High AoA because when we go out of control, there’s always a chance we don’t know what’s going to happen. It could be that the engine flames out because the intake wasn’t meant to spin sideways. And we’ve seen that before. As a result, we’re ready. All pilots flying High AoA did air starts earlier this year,” he continued.

While the F-35 ITF was well rehearsed and prepared to begin High AoA testing, the team had one more challenge to overcome before getting underway.

They had to successfully attach the spin recovery chute (SRC) to the aft of AF-4; giving pilots the capability to deploy the parachute and safely recover the aircraft in flight should the aircraft not be able to regain control during High AoA testing.

“This piece of equipment is a safety of test hardware; meaning that it’s critical for the testing of High AoA for safety reasons. We’re intentionally putting the aircraft into a nearly unrecoverable state of flight conditions and this piece of equipment is absolutely necessary if we do go out of control,” said Dillon Davis, F-35 flight test engineer from Lockheed Martin.

“The SRC is necessary to be able to recover the aircraft in flight. It’s necessary for safety of hardware, but more importantly safety for the pilot,” he continued.
Maintainers from the F-35 ITF encountered several unique challenges while performing the modification to the aircraft. While some of the problems were anticipated, the team remained unsure of their extent. Through creative ingenuity the F-35 ITF overcame challenges associated with executing the modification for the first time.

“We ran into a lot of first times with this modification and that always adds additional challenges when you’re doing something for the first time,” said Davis.
Contributing to the challenging circumstances, is the design of the F-35 and the fact that the SRC is considered flight test hardware and therefore not designed with long-term maintenance in mind.

“There were some inaccessible nut plates inside a fuel cell and when you break the nut plates off they were not accessible through any standard way. In the end, we ended up cutting holes directly into the skin of the aircraft, right into a fuel cell. That was not the only inaccessible area for nut plates. Another area only became accessible once the engine was removed,” said Davis.

Once the team integrated the SRC to AF-4, they were once again faced with a unique challenge and had to figure out how to fix the gaps between the SRC fittings and the aircraft.

“Once the SRC quadrapod was integrated on the aircraft, we found gaps between the SRC fittings and the aircraft structure. We had to figure out how to shim those gaps. We found some liquid shim, which is a tricky material to work with especially in the horizontal configuration. In fact, using this had never been done in a horizontal configuration for SRC fittings,” said Davis.

Modification for the aircraft began in early August and continued throughout the month of October until the team finally conducted the SRC taxi deployment test Oct. 20.

Although the integration of the SRC took longer than initially expected, the program made up valuable time with the recent successes of the High AoA testing.
“The testing is going very well; I’m extremely pleased with the progress. But, it’s important to note that we are finding areas for improvement. We are feeding that information back for follow-on software versions that will make the aircraft safer and effective in maneuvering at high angles of attach. By the time we get done, the aircraft will fly up to 50 degrees angle of attack with care-free handling qualities” said Schwartz.

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